1. Field of the Invention
The present invention relates to a DNA microarray (DNA chip) which specifically reacts with a biochemical specimen and which is used for inspection equipment represented, for example, by a biochip to be used in order to obtain information on the structure and the function of the specimen. The biochip preferably has several hundreds to several ten thousands kinds of captures, especially DNA fragments or the like, for capturing the specimen, and the captures are aligned and fixed at a high density as minute spots on a base plate such as a microscopic slide glass. The present invention also relates to a method for producing the same.
2. Description of the Related Art
The methods for analyzing gene structures have remarkably progressed in recent years. A large number of gene structures represented by those of a human gene have been identified. The analysis of the gene structure as described above uses a DNA microarray (DNA chip) in which several hundreds to several ten thousands kinds of DNA fragments or the like are aligned and fixed as minute spots on a base plate such as a microscopic slide glass.
The methods widely used for forming the minute spots for the production of the DNA microarray are generally based on a system such as the QUILL system, the pin & ring system, and the spring pin system in which a sample solution is stamped onto the base plate by using a pin.
Even when any one of the foregoing methods is adopted, it is important to minimize the dispersion of the volume and the shape of each of the minute spots so that the distance between the respective minute spots is maintained to be constant. Further, it is necessary that the capture (corresponding to the DNA fragment or the like in the case of the DNA microarray), which specifically reacts with a specimen in the minute spot formed on the base plate and which is used to obtain information on the structure and the function of the specimen, is reliably immobilized on the base plate.
On the other hand, in order to realize a higher density, it is also greatly expected to develop a new method which is excellent in productivity and in which the shape control performance for the minute spot is satisfactory.
In the conventional production of the biochip, when the sample containing DNA fragments or the like is dropped onto the base plate to form minute spots, or when the chip is dealt with or treated thereafter, the minute spots may be peeled off. When the DNA fragments or the like are immobilized on the base plate in the DNA microarray, several techniques are adopted, in which a functional group is affixed to the DNA fragments themselves to facilitate immobilization, or the surface of the base plate is coated with a functional group layer. However, even when such a technique is adopted, the adopted technique is insufficient to avoid the peeling off of the minute spots.
Further, a technique is also adopted, in which a hydrophilic polymer or the like is dissolved in a sample containing DNA fragments or the like to reinforce the immobilization of the DNA fragment or the like onto the base plate. However, the cost may be increased, because it is necessary to mix the sample and an immobilization-reinforcing solution, and it is also necessary to use a large amount of the immobilization-reinforcing solution. Further, when such an immobilization-reinforcing solution and samples are previously mixed and supplied onto the base plate, it is necessary to consider the compatibility and the conformability between the immobilization-reinforcing solution and the thousands of potential samples. Therefore, the selection of the material for the immobilization-reinforcing solution has been restricted.
Further, the sample containing the immobilization-reinforcing solution has to be supplied onto the base plate in accordance with the conventional method. Therefore, a drawback arises in that only an immobilization-reinforcing solution, which has physical properties to be successfully supplied by the supply method, can be used.
Specifically, in the method for mechanically forming spots using a pin to produce the DNA microarray, the sample containing the immobilization-reinforcing solution must be a sample which adheres to the pin, because the sample is physically retained (adhered) onto the pin to move (supply) the sample onto the base plate. Further, it is necessary that the amount of adhesion is made as uniform as possible for all cases of DNA fragments or the like. Therefore, the selection of immobilization-reinforcing agents that can be mixed with DNA fragments or the like has been extremely restricted.
On the other hand, an ink-jet type spotting method, which is one of non-contact type spotting methods, is known as a method for accurately forming minute spots. A micropipette in which a piezoelectric/electrostrictive element is used as a micropump and a dispenser which is based on the use of the same have been developed and practically used as an apparatus for accurately dispensing a minute amount of biological sample based on the use of the ink-jet type spotting method.
In the non-contact type spotting method, a biological sample, which contains, for example, DNA fragments, nucleic acids, and amino acids, is discharged as minute droplets so that the sample is dropped onto a base plate such as a slide glass.
However, in this method, when the minute droplets of the biological samples, which have relatively high viscosities, are of many different types, and differ only slightly in physical properties, are discharged to the space and dropped onto the slide glass base plate, so-called satellites (splashed droplets finer than objective discharged droplets) tend to appear in addition to the objective droplets (objective discharged droplets). The satellites dropped onto the base plate result in problems of quality of the obtained product. For example, such spots are formed at portions other than original spot formation positions making it impossible to maintain constant spacing distances between the minute spots and causing contamination due to the mixing with each other.
The so-called satellites as described above are not caused at the early stage of the operation of the dispenser in some cases, but the satellites occur after continuing the operation for a certain period of time. An extremely troublesome problem arises in this way in view of the management of production steps. When the discharge speed of the droplets is large, the momentum of the droplets is large upon the dropping onto the slide glass base plate, resulting in a problem such that splashes (mists) are generated forming satellites around the genuine spot.
In order to avoid the occurrence of the so-called satellites as described above, the discharge speed may be decreased. However, if the discharge speed is decreased, the discharge operation becomes unstable.
When the method, in which the hydrophilic polymer or the like is dissolved in the sample containing DNA fragments or the like to reinforce the immobilization of the DNA fragments or the like onto the base plate as described above, is adopted for the ink-jet type spotting method, the sample solution is discharged onto the base plate through a discharge nozzle. However, the solidification of the sample solution may be advanced during the process of discharging the sample solution onto the base plate, resulting in discharge failure. When an immobilization solution having a high viscosity is mixed with the sample to effect the discharge, any discharge defect is likely caused by the drying and solidification of the sample solution in the vicinity of the discharge nozzle.
Further, when it is intended to actually obtain information on the structure or the function of a specimen, the specimen may be bound to the base plate in locations other than the spots. Conventionally, in order to avoid such a phenomenon, a blocking treatment (treatment to avoid any binding of the specimen to portions other than portions at which the spots are formed on the base plate) is performed after forming the spots on the base plate. However, most of captures supplied onto the base plate may flow out during the blocking treatment. Further, the blocking treatment may be incomplete and the S/N ratio of the signal from the spot is deteriorated.